Optimization of the ASDEX Upgrade glow discharge

Abstract

The ASDEX Upgrade (AUG) system for glow discharge (GD) has been revised comprehensively. This was necessary due to technical as well as operational requirements. The available space with in the low field side of the vacuum vessel is strongly limited because it's the preferred place for diagnostics. The new development based on the design of the W7-X anode now has a smaller footprint, is long term reliable and easier maintainable. Additionally each of the anodes is equipped with a separate starting device. The use of this device now allows the fast ignition of GD at operational pressure without the need of a short-term pressure increase. With the change from carbon to tungsten plasma-facing components (PFCs) an enhanced Helium (He) content at plasma discharges was observed for unboronized walls and the implantation of He during GD could be identified as source. For this reason the use of He GD has to be strongly reduced. It turned out that pulsed GDs of about 10 seconds length followed by a pumping down phase are sufficient to clean the wall. This mode of operation requires a reliable breakdown of the GD at operating pressure. This paper gives an overview of the technical setup and first measurement results.

Wall conditioning in fusion devices with superconducting coils

“…Comparing different discharge duty cycles for GDC procedures with 5 minutes duration, it is found that 10-second-long discharges followed by 50 s for pumping are optimal. Retention of helium is reduced by 83.3% while removing still 68.7% of the hydrogen compared to continuous GDC operation [20]. The applicability of GDC for T-recovery in ITER is assessed by considering 5 operation days with a modest number of 10 pulses per day of 400 s each operated in JET.…”

Recovery from a hot water leakage at the tokamak ASDEX upgrade

“…On September 25, 2018, a Boronization [4], including an 8 h He glow discharge [6], was applied to optimize the wall conditions. Operation was started the next day.…”

Design and Application of Glow Discharge Cleaning at Uragan-2m Stellarator

“…A significant disadvantage is the sputtering of materials in contact with the plasma, which leads to the deposition of metal films on the equipment located in it [8,9]. Despite this drawback, the technology of cleaning the wall of the vacuum chamber with a glow discharge plasma is the most common and is used in large installations, for example, in TEXTOR [8], Wendelstein 7-X [9][10][11], ASDEX-U [12], JET [13], LHD [14], EAST [15], SST-1 [16]. In the future, it is planned to use the GDC system at ITER [17].…”

“…Common to all systems is the use of a vacuum chamber wall as a cathode, and specially designed electrodes as anodes, which are placed into the vacuum volume. The number of electrodes (anodes) is usually from 2 (SST-1 [16], LHD [14]) to 4 (ASDEX-U [12], JET [13], EAST [15]). The electrodes (anodes) 10, are installed on Wendelstein 7-X [9][10][11].…”

“…The electrodes are usually placed symmetrically along the torus. Stainless steel [15,16] or graphite [9][10][11][12] are used as the electrode material.…”